The properties of a semiconductor element are deteriorated due to variations in environmental humidity or temperature, and due to contact with fine dust or foreign matter. Also, semiconductor elements are readily damaged due to vibration or impact. In order to protect semiconductor elements from these external factors, a ceramic box or a resin for sealing is normally used as a semiconductor package.
In the case of a solid-state image sensor such as a CCD or a C-MOS, or a semiconductor element having an optical function such as a photo-diode and a photo-sensor, a transmission path for light is required between a semiconductor element mounted inside a package and the exterior of the package. Accordingly, in the case of a semiconductor element having an optical function, the periphery of the package cannot be entirely covered with a resin or ceramic. For this reason, semiconductor elements having an optical function are generally mounted at a hollow-type package having an opening, and the opening is sealed with an optically transparent window material such as glass for use as a semiconductor package.
In recent years, a large market for digital cameras has emerged. Together with increasing reductions in size and weight, and simplification in the operation of digital compact cameras, has come increased demands for improving image quality by enhancing expression of gradation rather than by increasing the number of pixels, and for employing special photographic techniques. Consequently, in recent years, the fields of development and commercialization of digital single-lens reflex cameras have become particularly active.
In digital single-lens reflex cameras, in order to realize high image quality, employ high-level camera techniques, and secure compatibility with lenses used in conventional single-lens reflex cameras that use silver salt films, a solid-state image sensor far larger than those used in digital compact cameras is used.
Table 1 is a list of representative CCD type solid-state image sensors that are used in digital compact cameras and digital single-lens reflex cameras. The size of the solid-state image sensors differ slightly depending on the manufacturer even when the same name is used; representative ones of each type are shown Table 1.
The name of the type of the image sensor mentioned in Table 1 are ones generally used for image sensors used in digital cameras. The size and area are actual measurement values of an imaging surface of the corresponding image sensor, the area ratio is a relative value of the area of the corresponding image sensor with respect to the area of a 35 mm size image sensor, which is regarded as 100, and the aspect ratio is a numerical value obtained by dividing the length of a long side of the corresponding image sensor by the length of a short side thereof.
The values mentioned here for the 35 mm size relate to values for conventional cameras that use silver salt films. However, these values are also adopted in a few high-end digital single-lens reflex cameras.
TABLE 1CameraImage SensorSize/Area/AspectTypeTypemm × mmmm2Area RatioRatioCompact¼Inch3.6 × 2.07.20.81.80Type⅓Inch4.8 × 3.617.32.01.33Camera½Inch6.4 × 4.830.73.61.33 1/1.8Inch6.9 × 5.235.94.21.33⅔Inch8.8 × 6.658.16.71.33Single Lens 4/3Inch17.3 × 13.0224.926.01.33Reflex-TypeAPS-C23.7 × 15.6369.742.81.52Camera35mm Size36 × 248641001.5
Even the smallest class of a solid-state image sensor used in a single-lens reflex camera, known as four-thirds ( 4/3 inch size), is about four times as large as the biggest class of an image sensor used in a digital compact camera (⅔ inch size).
In conventional silver salt film cameras, even when light beams entering through an imaging lens are incident obliquely on a silver salt film on which an image is to be recorded, photoreceptors present in a photosensitive layer of the silver salt film react to record an image without showing any difference from the case when light beams are incident vertically on the silver salt film. Accordingly, in silver salt film cameras, there is no significant difference in photosensitivity between the center of the film where the incident angle of light beams is vertical and a periphery portion of the film where the incident angle of light beams is oblique.
On the other hand, in digital cameras, a solid-state image sensor corresponding to a silver salt film that records an image is formed from arrays of many pixels. Each pixel has a micro-condenser lens disposed on an imaging lens side that condenses light onto a power-generating layer such as a CCD or C-MOS located at a recess of the pixel to generate charges, thereby recording an image. A micro condenser lens disposed to each pixel in a solid-state image sensor is designed so as to condense light beams in parallel with an optical axis of the condenser lens onto a power-generating surface. Therefore, light beams at an incident angle of equal to or more than the critical angle to a micro condenser lens of each pixel are condensed onto a side surface of the pixel, rather than onto the power-generating layer at the recess of each pixel, thereby failing to record an image.
This problem is known as oblique incidence characteristic of an imaging sensor, and it is more serious with an imaging lens having a large image angle or a pixel located at a periphery position of an image sensor, since the reduced amount of light in a peripheral portion of an image degrades the image quality thereof.
Here, an incident angle means an angle formed by a normal line to a surface to which light is incident and the incident light. When light is incident vertically to the surface, the incident angle is 0°. The more obliquely the light is incident, the larger the value of the incident angle is.
In the case where an imaging lens is not exchanged, as with the case of a digital compact camera, it is possible to design a camera by considering the characteristics of an imaging lens and a solid-state imaging sensor as a whole so as to restrict an imaging angle of an imaging lens to a range in which the reduction in the amount of peripheral light is acceptable. Furthermore, when the size of the solid-state image sensor used is small, since an image circle of the imaging lens is small and the incident angle at the pixels positioned at the periphery of the solid-state image sensor is not increased, the amount of light in a peripheral portion of an image does not decrease.
On the other hand, in a digital single-lens reflex camera, an imaging lens is exchanged by a user to one with a desired image angle. Therefore, it is necessary to design the camera so that decrease in the amount of light in a peripheral portion of an image may be within an allowable range, even when an ultra-wide angle lens with the largest image angle or a fisheye lens is used. Furthermore, the larger the size of the solid-state image sensor is, the larger the incident angle of light beam to pixels in the peripheral portion is, and the light amount tends to decrease.
Accordingly, in a digital single-lens reflex camera that uses a large solid-state image sensor, as compared with a digital compact camera, desired dimensional accuracy of a resinous hollow package that mounts a solid-state image sensor is significantly stringent. In particular, when the flatness of a mounting surface for a semiconductor element is poor and the solid-state image sensor is obliquely mounted to the resinous hollow package, reduction in the amount of light in the peripheral portion may occur in an uneven manner. Namely, in pixels in the peripheral portion of the image sensor on an uplifted side from the mounting surface for the semiconductor element, the incident angle of incident light is decreased and reduction in the amount of light is less likely to occur. On the other hand, in pixels in the peripheral portion of the imaging sensor on a subsiding side, the incident angle of incident light is further increased and the amount of light is reduced to a degree that exceeds a designed value.
There is a demand for digital single-lens reflex cameras to ensure operation even under conditions that are so severe that a digital compact camera cannot work properly, such as those of news photography or mountain photography. In order to secure operation of a solid-state image sensor, moisture resistance is particularly important among various factors. In order to address the above problem, Japanese Patent No. 2539111, for instance, discloses a technology of providing a moisture-proof island that inhibits moisture penetration under a semiconductor element mounting surface of a resinous hollow package on which a solid-state image sensor is mounted.    Patent literature 1: Japanese Patent No. 2539111